The present invention relates to papermaking machines used in forming tissue grades particular.
Tissue is a lightweight grade of paper which is formed, pressed and dried on a single large dryer called a Yankee dryer. The tissue is creped from the surface of the Yankee dryer by a doctor blade creating a soft absorbent tissue. Tissue is widely used in products such as toilet paper, paper towel, napkins, and facial tissues.
A typical tissue forming machine consists of a fourdrinier former which transfers the web to a press felt which conducts the tissue web through a pressing section. Following the pressing section the web is pressed against a Yankee dryer roll to firmly adhere the web to the surface of the roll. The transfer roll may have a nip loading of approximately 200 pounds per linear inch and sometimes various sprays are used to increase adherence to the surface of the Yankee dryer roll. The tissue web is rapidly dried on the surface of the Yankee dryer which may have a diameter of over 22 feet. The dried web is scraped off the surface of the dryer by a doctor blade in a process known as creping which increases the bulk and absorbency of the tissue.
When attempting to modernize a typical older tissue making machine, the forming section, which has limited water handling capacity, which impacts formation potential, and needs substantial upkeep due to age, is advantageously replaced with a modem twin wire former or crescent former. Updating the pressing section, however, presents problems: modem fabrics are stiff and cannot be bunched up and threaded through the threading openings on older machines.
The existing pressing section typically cannot sufficiently reduce the water content of the higher speed web produced by the new crescent or twin-wire former. Because drying capability is limited by the maximum size and temperature of the Yankee dryer, if the performance of older machines is to be increased without increased drying capability, a solution which improves the capacity of the pressing section must be found. At the same time, it is necessary to eliminate the long spans where the web is supported by a felt, which have the potential for sheet drop-offs and hole formation.
What is needed is a combination of tissue forming section and press section which can be used to improve the performance of existing tissue making machines.
The papermaking machine of this invention forms a web of tissue in a crescent former, followed by an air press for water removal in the pressing section. The web is then pressed onto a Yankee dryer for drying. The crescent former employs a breast roll around which a upper fabric and a lower fabric are brought together. A headbox injects a stream of stock between the two fabrics as they are brought together on the breast roll. The upper fabric engages the breast roll first, and water is driven off through the lower fabric due to centrifugal acceleration as the wires move together around the breast roll. The two forming fabrics move together over a series of vacuum boxes, and the sheet is heated with steam and passed between an upper pressure box and a lower vacuum box forming an air press which dewaters the web by forcing air through the web. A sheet transfer pickup vacuum box holds the sheet to the upper fabric as the lower fabric diverges away from the upper fabric. A pressure roller transfers the web onto the surface of a Yankee dryer. A tail cutter is located between the sheet transfer box and the pressure roller. Each of the fabrics has a vertical run of fabric before it returns to the breast roll. The vertical runs allow the forming fabrics to be cleaned while baffles prevent the cleaned fabric from being sprayed.
Various modifications can be made to the basic design. A second upper fabric or transfer fabric operating at a lower speed then the first upper fabric can be used to form a rush transfer between the forming fabrics and the Yankee dryer which increases web bulk and absorbency. Instead of a second fabric, a vacuum pressure roll may perform the rush transfer between the lower forming fabric and the Yankee dryer.
The lower fabric may be divided into two fabrics, and the second fabric which passes through the air press, can be used to mold a texture into the web.
The rush transfer and the molding fabric maybe combined in one machine.
A twin wire former may be used instead of a crescent former in each of the above configurations.
It is a feature of the present invention to provide a replacement forming and pressing section to an existing papermaking machine for forming tissue.
It is another feature of the present invention to provide a pressing section of shorter length.
It is a further feature of the present invention to provide for cleaning forming fabrics as they move upwardly along a vertical path.
It is a further feature of the present invention to increase the speed at which a tissue forming papermaking machine operates.
It is yet another feature of the present invention to provide means for integrating an air press into a papermaking machine for forming tissue grades of paper.
A yet further feature of the present invention is to provide an improved tissue former which utilizes portions of an existing machine and thus reduces costs.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.